1. Field of the Invention
The present invention relates to a stepping motor unit of the type wherein the internal space of a stepping motor is filled with an oil such as a silicone oil so as to prevent the stepping motor from falling out of step and generating undue heat. It also relates to a rotary control valve incorporating such stepping motor unit.
2. Description of the Prior Art
It is known that out-of-step and abnormal heating of the stepping motor can be substantially avoided by filling the internal space of a stepping motor with a silicone oil or the like.
The silicone oil thus filled in the internal space of the stepping motor is effective to lower or dampen the inertia of a rotor when the stepping motor is coming to a stop after completing a step or slew motion. Thus, out-of-step of the motor resulting from overshoot is prevented to a certain extent. Furthermore, the silicone oil is also effective to prevent abnormal heating which would otherwise occur in a stator winding of the motor. Consequently, the continuous service time period and the service life of the stepping motor are substantially extended.
One such known stepping motor is disclosed in Japanese Utility Model Laid-open Publication No. 59--177876 in which the stepping motor is combined with a control valve unit to jointly constitute a motor-operated flow control valve for controlling the rate of flow of a hydraulic oil which also fills the internal space of a housing in which the stepping motor and the control valve unit are sealingly received.
When the motor-operated flow control valve is used in an automobile or the like application in which the environmental temperature varies widely, the foregoing out-of-step protection is difficult to achieve because the viscous condition of the hydraulic oil in the motor varies with the environmental temperature. For instance, when the environmental temperature is very low, the viscosity of the working fluid is considerably high with the result that only an insufficient motor torque is produced. Conversely, when the environmental temperature is very high, the viscosity of the hydraulic oil has a very low and hence has an insufficient inertia damping effect which allows the stepping motor to fall out of step.